The present invention relates generally to optical polarizers, and more particularly to integrated in-line fiber optic polarizers.
There are a variety of optical applications which require polarized light. For example, optical gyroscope applications require one defined polarization in order to satisfy reciprocity conditions. Likewise, optical coherent communications systems require the use of polarized light in order to combine the light coherently.
Ordinary single mode optical fibers cannot hold a given polarization state of light, i.e. maintain the state of linear polarization along a defined axis. This is because in such a fiber the two polarization modes have very close propagation constants so that it is very easy to couple therebetween with the slightest perturbation. Accordingly, polarization holding optical fibers must be utilized in applications requiring polarized light. Currently, such polarization holding fibers are implemented by means of highly birefringent fibers or by means of the use of an elliptical fiber core. In the first case, a strain is placed on the fiber core so that the index of refraction is different in orthogonal directions. This difference in the index of refraction for orthogonal directions leads to different propagation constants for those directions. The polarization holding fiber acts to maintain a given input polarization with very little coupling between orthogonal polarization modes. However, both polarization directions will still be propagated in the fiber. If polarized light, i.e., light with a single polarization mode, is required, then some form of optical polarizer must be inserted into the optical fiber line. It is current practice to use bulk optic polarizers to effect this polarization function in the optical fiber system. Such bulk optic polarizers, for example, calcite crystal polarizers, are typically relatively large (on the order of one-half cubic inch) and require precise alignment in the system. These bulk optic polarizers also are subject to mechanical vibration and have a high insertion loss (output power/input power), thus increasing the attenuation on the polarization mode to be transmitted in the line. Accordingly, such bulk optic devices are not practical or desirable for optical fiber systems.
A number of polarizers which are integrated into the optical fiber line have been proposed in the art. The reference "Single-Mode Fiber-Optic Polarizer" by Bergh, Lefevre, and Shaw, Optics Letters, Nov. 1980, Vol. 5, No. 11, pp. 479, discloses the replacement of a portion of the fiber cladding with a birefringent crystal to couple unwanted polarizations from the optical fiber. Note that only an ordinary fiber is utilized in this reference. The reference "Single Mode Fibers With Asymmetrical Refractive Index Pits On Both Sides Of Core" by Hosaka, Okamoto, Sasaki and Edahiro, Electronics Letters, Mar. 5, 1981, Vol. 17, No. 5, pp. 191, discloses a polarizer with asymmetrical refractive index pits on both sides of the core. This side-pit fiber supports only a single polarization mode and propagates the other mode with very high loss. However, this side-pit polarizing fiber is difficult to manufacture in order to obtain the single-mode polarization.
A major difficulty with the foregoing integrated optical fiber polarizers is that they are not truly "in-line" with the polarization-holding optical fiber. These prior art fiber polarizers must be connected with a splice or fusion joint at which precise alignment is required. This alignment requirement creates a significant fabrication problem and thus makes such fiber polarizers impractical for optical systems.